To adjust a vehicle seat or individual components of the vehicle seat, for example the backrest, the seat cushion, the headrest or the entire seat or seat frame, direct current motors are especially used, which are actuated via a control device.
Further, in addition to adjustments for comfort at a low adjustment speed, pre-crash adjustments are also known, in which generally a higher adjustment speed is employed. In this connection, the control device ordinarily receives a rapid adjustment signal from a sensor. Adjustments of this type are known, for example, from WO 01/45979 A1, DE 199 61 799 A1, and DE 197 49 838 A1. In these, sensors in the vehicle, such as distance sensors, detect an imminent crash state, such as a collision with an obstacle located in front of the vehicle, and the control device transmits a control signal to an adjustment device, so that the adjustment device can move a seating component to a crash-safe position before the collision.
With an adjustment of this type, a greater degree of crash safety can be ensured in many situations. Nevertheless, frequently the amount of time available for adjusting the vehicle components once the pre-crash situation has been detected is insufficient, especially if one's own or another's vehicle is traveling at high speed, or if the performance capacity of the adjustment device is too low.
Permanently excited direct current motors, with their light weight and low manufacturing costs, offer high performance capacity even at low currents. Ordinarily they are designed such that the high motor current that is achieved during the start-up of the direct current motor with application of a high voltage will not result in an unacceptable, irreversible demagnetization of the permanent magnet. With permanently excited direct current motors, the maximum amount of power that can be accommodated in a given installation size is limited by the magnets used and by the maximum permissible magnetic field. If the critical value of the magnetic field is exceeded, an irreversible demagnetization of the permanent magnet will result.
The critical operating point is at start-up, as at this point the greatest amount of current, the so-called block current, is flowing. This operating state is particularly critical at low temperatures, when the resistance of the copper winding is at its lowest, and the current, and therefore the magnetic field as well, are at their highest. For each predetermined motor geometry, these boundary conditions result in a maximum realizable performance. Accordingly, the direct current motors are designed such that at the time of start-up, no excessive block currents can occur.
For this reason, the control range of the permanently excited direct current motors in seat adjustment mechanisms is limited. Thus, in order to achieve high adjustment speeds for pre-crash adjustments, direct current motors having larger dimensions, which are designed for higher block currents, generally must be used. Accordingly, high manufacturing costs and large mounting volumes result.